Place potatoes, half the scallions, 1 T. salt, and enough water to cover in large heavy pot. Cover and heat to boiling. Remove cover and cook over medium heat until potatoes are tender, 20-25 minutes. Drain, reserving 1/2 cup cooking liquid. Mash potatoes. Return potatoes to pot.
Stir in reserved cooking liquid, the cream, butter, all but 2 t. of the remaining scallions, 1/2 t. salt, the pepper, and nutmeg. Transfer to warmed serving dish, sprinkle with remaining scallions, and serve hot.